What We Know About the Theory That Venus Ate Its Moon

Venus is nearly Earth's twin in mass, size, and structure, yet it has no moon. A study led by University of California, Riverside astrophysicist Steven Kane proposes that Venus once did have one — and that the planet's own gravity and rotation pulled the satellite inward until it broke apart and was absorbed, according to WIRED.

What does the new study claim happened to Venus's moon?

Instead of a moon being destroyed by a collision with another celestial body — a previously proposed explanation — the UC Riverside team argues no catastrophe was necessary. "Venus didn't require a catastrophe to arrive at what we can see today," Kane said in a press release cited by WIRED. "It turns out the gravity of the planet itself combined with the rate at which it spins naturally caused the moon to collapse on top of it."

Why would Venus's slow rotation pull a moon inward instead of pushing it away?

A planet's spin affects whether it pushes or pulls its satellites through variations in gravitational strength, per WIRED's report. Earth's roughly 24-hour rotation is fast enough that the moon drifts outward, at about 1.5 inches per year. Venus is different: it takes 243 Earth days to complete one rotation, longer than its own year and the slowest spin rate of any planet in the solar system. That sluggish pace, combined with the planet's gravity, means a moon orbiting Venus would be dragged inward rather than pushed away.

How did researchers test the theory?

Kane's team built a computer model simulating gravitational interactions between celestial bodies, then validated it by reproducing the known evolution of Earth's moon, WIRED reported. Researchers then ran simulations varying Venus's rotation speed and the mass of a hypothetical moon, testing values from 0.01 to 10 times the mass of Earth's moon. The results showed the moon's fate hinged on rotation speed, lunar mass, and orbital shape — and in most scenarios, the satellite spiraled inward, fragmented, and was consumed by the planet.

Could a moon have survived under different conditions?

In a limited number of test cases, survival was possible. When researchers artificially sped up Venus's rotation to once every 10 to 12 hours and gave the moon a mass comparable to Earth's, simulations showed a moon could persist for up to 4.5 billion years, according to WIRED. But that scenario bears little resemblance to Venus's actual, far slower rotation today, making it an interesting stress test of the model rather than a plausible history.

Does this prove Venus once had a moon?

No. WIRED reports the study does not establish that Venus ever had a moon in the first place — it offers a mechanism for how one could have disappeared if it existed. Kane called the consistency of the outcomes surprising: "When I made this discovery, I was shocked. I thought surely the broad range of scenarios I was exploring would lead to a variety of results. But it all went pretty much in the same direction." Confirming the theory would require physical evidence, which WIRED notes will be difficult to find given Venus's hostile surface conditions.

By the numbers

• 243 Earth days — time for one full rotation of Venus, longer than its own year
• 1.5 inches per year — rate Earth's moon currently drifts outward
• 0.01 to 10x — range of hypothetical moon masses (relative to Earth's moon) tested in simulations
• 10–12 hours — artificially sped-up rotation rate used in the one scenario where a moon survived
• 4.5 billion years — how long a moon could persist under that altered, faster-spin scenario